The invention relates to electronic flash apparatus for photographic purposes, having an exposure-measuring and flash-limiting device which comprises a light-sensitive element connected in series with an integrating capacitor and a thyristor which, when it becomes conductive, causes a pulse which effects the interruption or termination of the flash. Photographic flash apparatus of this general type, having the features just mentioned, is well known in the art.
In one of the known forms of this general type of flash apparatus, a series connection of a photo-transistor and an integrating capacitor is connected between the plus and minus poles of the voltage source of the exposure-measuring and flash-limiting device or mechanism. The control electrode of the thyristor is connected to the point of connection between the emitter of the photo-transistor and the integrating capacitor, and the cathode of the thyristor is connected with the adjustable resistor of a voltage divider. When a specific capacitor voltage is reached, the thyristor is ignited (i.e., becomes conductive) and the flash discharge is broken off or terminated in known manner. The ignition threshold of the thyristor can be adjusted by the variable resistor connected to its cathode.
In this known form of device just mentioned, it is found that the electronic flash apparatus switches off (i.e., the flash is terminated) with considerable time delay when it is used in the close-up range, that is, in the case of a relatively short distance between the flash apparatus and the subject being photographed, so that the photographic images are always over-exposed. Therefore it has been proposed to improve this situation by connecting the cathode of the thyristor both to the center cap of a voltage divider circuit which is energized only during discharge of the flash tube, and also through a capacitor to a lower potential point of this voltage divider circuit. In this way, the cathode potential of the thyristor increases in time-dependence in a predetermined manner in a short time range after ignition of the flash tube, whereby the threshold voltage of the capacitor necessary for the ignition or firing of the thyristor is reduced in this short time range. The ignition pulse of the interruption or termination of the flash thus takes place slightly earlier than is required by the quantity of light measured by the photo-transistor, and this in turn compensates for the over-exposure which would otherwise occur when making close-up pictures.
Although this arrangement just described was apparently a step in the right direction, experience has nevertheless shown that this arrangement has the effect of making the adjustment of the exposure-measuring and flash-limiting mechanism more difficult. Moreover, the apparatus becomes more trouble-prone, so that careful attention must be given to a mutually screened spatial arrangement of pulse-generating circuits within the apparatus.
The present invention attempts to provide flash apparatus which overcomes these difficulties, and which is less delicate or trouble-prone, and more easily adjusted, as compared with the prior devices of the kind above mentioned, yet still containing the desirable feature of compensating for close-up photographs so that over-exposure does not occur.